It's like wheels within wheels. On the outside rim, skittering drum beats stutter madly at 160bpm, though there's never a simple, metronomic regularity to their measures. Instead, the snare patterns remain doggedly unorthodox, the machine's virtual drum-stick rarely falling in exactly the same place twice, as if obeying a kind of indeterminacy principle. On the next rim, the beat is halved to 80bpm, propelled by a fat synthesized bass, and this is the tempo at which people dance, if they dance at all. Further in still come odd samples and bits and bobs of this and that, snatches of movie dialogue, spacy synth chords, stereo pans, crackly vinyl surface noise leading to a battered Fender Rhodes loop - almost anything will do, if it works. Further in than that, deep in the belly of the music, is the very heart of darkness: a loping reggae pulse and sundry dub effects. Or perhaps the slower-than-slow ghost of a slack trip-hop rhythm. The sound of Bristol jungle - or, more fashionably, "drum and bass" - is unlike anything the rest of the country is producing, and it's about to go global.

They're talking about Roni Size in Japan, in New York and LA. Though his debut Talkin' Loud album is still not released (it should be out sometime in June), he's already an underground dance-music legend and together with his compatriot producers on the Full Cycle record label, such as DJ Krust and DJ Die, he's part of a movement that is arguably the most important new musical force of the moment.

Its roots reach back into the golden age of Bristol hip hop, pre-dating the emergence of Massive Attack, Tricky and Portishead, when in 1989 Krust and his brother Flynn (now half of Flynn and Flora, another brilliant Bristol drum and bass team, whose Native Drums album is just out on the Independent Dealers label) were part of the Fresh Four, whose Smith and Mighty-produced version of the old Rose Royce hit "Wishing on a Star" made the Top Ten and brought them a deal (albeit an abortive one) with Virgin. At Smith and Mighty's studio in Redland, the More Rockers label also puts out distinctive drum and bass releases, and revisionist dub- meisters Henry and Louis re-invent the sound of Lee "Scratch" Perry 20 years on.

At Full Cycle headquarters at Unit 23, Easton Business Centre, a shiny government-funded bulwark against the inner-city deprivation of downtown Bristol, the phone never stops ringing and the various mobiles of the principal partners are constantly pressed into service. Outside, a cameraman for a Channel 4 documentary bids Roni and Krust drive their battered saloon through the gates again and again to get a usable take. Inside, the sound of a trademark drum-loop jerks away unattended as the DJs get themselves ready for the weekend's trip to Amsterdam. "We've been to all kinds of countries," Krust says. "France, Germany, Vienna..."

Their schedule is fuller than full; for two years Full Cycle has been putting out a single a month - a selection of which are collected in the new album Music Box: a new era in drum and bass - and until recently, when it was acrimoniously axed, Roni and Krust did a weekly radio show on the local station Galaxy. The show became required listening, especially in the region's prisons whose residents' letters filled the post-bag each week. Forced into an interview, Roni - a small young man with long, braided hair tied back into a ponytail - sighs and begins to spiel, shutting the door of the studio against the ringingphone and the constant swearing of Krust.

"For me, what I was doing before, I can't really remember," he says. "I just remember making music and then people calling it jungle and then drum and bass. You used to have English rap groups trying to identify with American groups but it wasn't for me. I used to like the beats. But when it came to the lyrics I wasn't into none of it, so I moved on to instrumental music, and it was minimal and we were like minimalists. I liked reggae and I used to like the warmth of the bass so I took the breaks from hip hop and the bass from reggae and sped the rave up, so you come up with a different formula, and then it became English."

He started to work with reggae sound systems after being thrown out of school and began learning about recording at the Basement Project, a youth club-cum-community music facility that has become an important force in training up Bristol producers.

Flynn and Flora started to formulate their own form of jungle after travelling out to free parties as a sound system in the early Nineties. "Everybody was trying that hip hop thing so hard," Flynn says, "but it couldn't quite make it because it was so American. And then rave came out and suddenly boom! - everyone could get it, there were no lyrics and you could throw in all of these samples. The break-beat thing went into drum and bass while the kick thing went into heavy house and hardcore."

They work with primitive equipment in their home, at the kitchen table, sampling as they go. "It's very minimal," Flora says, "very low budget. It's really weird because everyone else seems to have the right samplers, everyone has Akais or whatever, mad machines that can do everything, whereas ours is ancient - the sampling time is about 14 seconds."

"We take anything from anything," Flynn adds, "videos, tapes from friends, old records." Their album includes a wonderful snatch of alto sax, perhaps Charlie Parker, and a dialogue excerpt that derived from mistakenly plugging their sampler leads into the television instead of the video. "We'd got the sample all lined up," Flora explains, "and then when we pressed the button this voice saying 'You can pull this switch' came out from the telly. We just thought, yeah, we've got to use that."

Back at Unit 23, Roni describes the lofty aims of the music. "When we were making Music Box, me and Die used to call it ozone-friendly music, meaning that everyone would like it. The beats wouldn't be too aggressive, the bass would be warm and melodic, the sounds would be universal. We used to sit and say to each other 'this is crossover music, you've got to like this'."

The much-touted jazz potential of jungle (though a forthcoming Jazz Jungle album on Acid Jazz manages to fall between both stools) is, for Roni, a matter of attitude. "To me, jazz isn't a music, jazz is a progression and we're always progressing, we never use the same sound twice." The layering process by which they assemble their productions is also more intuitive than rationalised. "You don't realise that you're doing it," he says. "You're just collecting all your favourite sounds, getting vibes from them and putting them in. If they fit, they fit. If not, you save them for another day. It's just experimenting and seeing what happens when we press a button. It's nothing out of the ordinary, just the simplest things. It's minimal, and it comes from experience, just like driving or learning to eat with chopsticks..."

All of them have a vision. He points through the window at a high- rise towerblock in the distance. "I want a building the size of that," he says. "A whole industry, with studios, cutting rooms, distribution, a bar and restaurant for the people who work there." He points up at the enormous Tannoy speakers squatting heavily on a rack close to the ceiling of the studio. "They cost five grand," he says proudly, "and we paid for them ourselves."

'Music Box - a New World of Drum and Bass', compiled by Roni Size, is on Full Cycle Records; 'Native Drums' by Flynn and Flora is on Independent Dealers, distributed by Vital; Phil Johnson's book on the Bristol sound, 'Straight Outa Bristol!', will be published in the autumn by Hodder and Stoughton